adding CMIP variable documentation

[duvivier]

Documentation about addition of CMIP6/SIMIP variable inclusion in history variables. Follow up to PR #191

• Developer(s): Alice DuVivier

• Please suggest code Pull Request reviewers in the column at right.

• Are the code changes bit for bit, different at roundoff level, or more substantial? bfb

• Please include the link to test results or paste the summary block from the bottom of the testing output below. n/a

• Does this PR create or have dependencies on Icepack or any other models? no

• Is the documentation being updated with this PR? Y - follow up to Update CICE with CMIP changes. #191
  If not, does the documentation need to be updated separately at a later time? (Y/N)

Note: "Documentation" includes information on the wiki and .rst files in doc/source/,
which are used to create the online technical docs at https://readthedocs.org/projects/cice-consortium-cice/.

• Other Relevant Details:

[dabail10]

I think this looks good. Thanks for taking care of this! It would be nice to point to the SIMIP paper on this here:

https://www.geosci-model-dev.net/9/3427/2016/

[duvivier]

@dabail10 I added the correct reference for SIMIP you suggest.

@eclare108213 @dabail10 (@proteanplanet ). I noticed some inconsistencies/duplicates in the bibliography while adding the above reference. I have also fixed these and the relevant citations in text. We have two references in master_list.bib that are unused anywhere in the documentation- Roberts2018 and Irving2015. It appears these were added as part of #43 from last November and are related to testing developed by Andrew but was a commit by @dabail10.
@proteanplanet , if you are able to quickly peruse the section that was added with #43 (https://cice-consortium-cice.readthedocs.io/en/master/user_guide/ug_testing.html#two-stage-paired-thickness-test) and check if these references are indeed missing and should be added, that would be helpful.

Finally, I just cleaned up some other formatting bugs that were giving errors in sphinx and causing minor oddities in the documentation.

[dabail10]

Looks good. We'll see what Andrew has to say about those references.